In the past, reducing machines, including pulverizing systems, have used disc mill assemblies to grind, shred or pulverize various types of materials such as plastics, nylons, polyesters and other polymers into powder, amongst other industrial applications. Typically, reducing machines have cooperating cutting discs having opposed cutting surfaces. Typically, one cutting disc is stationary, often referred to as the stationary disc, and one cutting disc is rotating, often referred to as the rotating disc. Input material to be reduced passes between the cutting surfaces of the discs radially from the centre to the circumference by virtue of centrifugal force, often assisted by a vacuum created by a fan forming a part of the reducing machine.
A major problem with reducing machines in general is the management of heat. As the input material is ground, shredded or pulverized by the relative rotation of the cutting discs, heat is generated and must be dissipated to avoid damage to the discs as well as potentially melting or degrading of the input materials. To facilitate cooling of the disc assembly, prior art reducing machines have generally utilized a water cooling system, including a water jacket assembly, for cooling the stationary disc as disclosed for instance in U.S. Pat. No. 8,282,031 B2 to Sly. The water jacket cooling assembly would permit water, or another liquid, to be circulated on the non-cutting surface of the stationary reducing disc to dissipate heat generated by the cutting surfaces of the disc assembly, and in particular the stationary disc when it is in facing operative relation the rotating disc arranged.
However, water jacket assemblies can be rather expensive to design, build and maintain, thereby increasing the cost of the overall machine. Also, water jackets leak regularly thereby causing rusting of the disc assembly, and/or contaminate the input material being reduced.
A further difficulty with water cooling of the stationary disc is that, invariably, the temperature of the stationary disc near the water inlet will be lower than the temperature of the stationary disc at a location remote from the water inlet due to the fact that the water will absorb heat while it is circulating and in thermal contact with the stationary disc. This can cause temperature variations and thermal imbalances in the stationary disc which can cause structural stress.
Furthermore, if the operators of the reducing machines are not careful and turn on the water cooling system when the stationary disc has been operating for some time and is at an elevated temperature, the stationary disc could experience “thermal shock” from a sudden temperature decrease. This often results in damage to the stationary disc and, in some cases, a catastrophic failure of the stationary disc.
Furthermore, because of the risk of “thermal shock” and other damage that could be caused by water cooling, the material used for the cutting discs, and in particular the stationary disc, would need to be selected such as to decrease the possibility of such “thermal shock” for safety purposes. In particular, the material of the stationary disc would need to be of a softer material to decrease the possibility of cracking.
A further disadvantage of the prior reducing machines is that considerable time is required in which to initially heat up the reducing machine prior to use. Typically, the reduced material generated while the reducing machine is warming up, is often called “off-spec” or “off specification” reduced material, and is usually discarded or blended back with the input material for further processing. At present, many prior art reducing machines are run with material for about 20 to 30 minutes in order to heat the reducing machine prior to producing useful reduced material. During the initial heating process, raw material is inserted into the machine and then the resulting off-spec material is discarded. Throughout the initial heating process, the stationary disc must be continuously cooled using the water cooling system, otherwise thermal shock could arise if the water cooling is suddenly commenced after the reducing machine, including the stationary disc, has been heated to an operating temperature. Because of this, the water cooling acts against the initial heating of the reducing machine thereby lengthening the amount of time required in order to heat the reducing machine to a useable temperature and generating additional off-spec material that is generally discarded or blended back with the input material. This also increases the wear and tear of the mill assembly as a whole because it must be operated for a longer period of time to heat the reducing machine.
Another disadvantage with prior art discs, and in particular rotating discs, is that cracks may develop, which could eventually lead to a failure, and eventually a catastrophic failure. While cracks may appear in both the stationary disc and the rotating disc, crack development and propagation are more common with rotating discs because of the increased stress caused by the rotation. Cracks can develop particularly near openings or orifices because of increased localized stress levels. Therefore, for safety concerns, it is important to decrease crack generation and propagation, particularly near openings or orifices in the rotating disc.
In addition, while rotating discs are cooled as a result of their rotation, this air cooling is often inefficient. This is the case, in part, because the rotating disc is often contained within a structural member, such as a carrying plate, which inherently insulates the rotating disc. In other cases, even if the rotating disc may be exposed to the air, the air is not efficiently channelled over the rotating disc. Furthermore, prior art devices may recirculate heated air within the disc chamber, decreasing cooling efficiency.
Furthermore, heat generation is a limiting factor of most reducing machines. Increased heat generation limits productivity and, conversely, increased heat dissipation increases productivity. Furthermore, increased heat generation limits the types of material which can be reduced.
Accordingly, the prior art reducing machines suffer from several disadvantages related to the manner in which the mill assembly, and in particular the stationary and rotating discs, are cooled. Furthermore, the method of cooling of the mill assembly, and in particular the stationary disc according to the prior art assembly, increases the cost of manufacture, assembly and operation and also restricts the nature of the material used for the discs.